Today has been the kind of day that is the reason we got into art and science. Bill, I got beautiful 120 film. If I could add the exposure numbers on the side, I don't think you could tell it was homemade. I loaded up an old Kodak Brownie Target Six-20 and got seven exposures (2-1/2 x 3-1/2). Someone with longer arms than I have could get the full eight, but I'm satisfied . And, I suppose if someone could figure out sprocket holes, they could shoot 35mm, but that won't be me. I've never been much of a 35mm fan.
I also got a great 8"x10" negative, and there's no reason you couldn't go ULF. Give or take a bit of technique tweaking for specific sizes, coating is coating. KIrk, this will be something Don will want to try out.
hrst, As I understand the history of film, cellulose acetate 'safety film' succeeded cellulose nitrate film (the nasty-fumed, burn up with a big bang stuff). Triacetate succeeded diacetate because it was more durable. Unfortunately, they found out that triacetate disintegrated in hot, humid conditions with the distinctive smell of vinegar. Polyester succeeded triacetate with great success (if you don't count broken film projectors) but I can't sub polyester. After my results today, it's no longer an issue for me. Although diacetate isn't as strong as polyester, it also doesn't scratch easily or pick up static dust. It will be my negative material for a long, long time to come. That's certainly not to say I don't wish you well on your poly subbing efforts. Good luck!
Tomorrow I'll write up a brief blurp on all this for The Light Farm before I go on an extended art hiatus -- if anyone is interested in further information.